The present invention relates generally to the reinforcing of a preformed cellulosic board by providing it with a lignin coating followed by a strength increasing calendering operation, and more particularly to increasing the crush strength of a cellulosic linerboard by treating it with a coating of lignosulfonate and then subjecting the coated linerboard to at least one calender nip.
Improvements in the physical properties such as increases in the compressive or crush strength of linerboard as used in the fabrication of corrugated products have been previously provided by coating the linerboard with a thermosetting resin such as urea-formaldehyde. However, it is desirable to limit the release of formaldehyde vapors into the atmosphere during the manufacture of the strength-improved linerboard and the end use of such linerboard. Government regulations pertaining to the use of formaldehyde can be expected to reduce the use of urea-formaldehyde as an adhesive and as a material for improving the strength properties of cellulosic products such as linerboard.
Recent developments have shown that lignin, such as in the form of spent sulfite liquors produced as waste during sulfite pulping processes, can be utilized for providing an adhesive or binder for the manufacture of cellulosic products such as particleboard. For example, as described in "Lignin-Based Wood Adhesives" by H. H. Nimz in the publication, Wood Adhesives, Chemistry and Technology, edited by A. Pizzi, published by Marcel Dekker, Inc., New York, N.Y., (1983) pp 247-288, spent sulfite liquor, which contains about 50 to 60 percent lignosulfonate, can be used as an adhesive in the manufacture of particleboard by cross-linking the lignin molecules in the lignosulfonate by condensation reactions using strong mineral acids at elevated temperatures or by oxidative coupling reactions using an oxidant such as hydrogen peroxide in the presence of a catalyst such as sulfur dioxide or potassium ferricyanide. By using such a lignin cross-linking procedure, a formaldehyde-free adhesive is obtained which can be satisfactorily used in place of the urea-formaldehyde resins previously utilized in the manufacture of particleboard and the like.
Efforts have been made to utilize lignin in place of the thermosetting resins, especially urea-formaldehyde, in treating linerboard to improve the strength properties thereof. It was found that some degree of success was achieved in increasing the strength properties of linerboard by using such treatments in that the cross-machine direction (CD) crush strength of the lignin-coated linerboard was found to be about 10 percent higher than that provided by a similar linerboard without the lignin coating thereon.
A resin reinforcement for linerboard should be such that it does not interfere with the repulping of the treated linerboard or corrugated boxes made from the treated linerboard. Cross-linked polymers, such as the urea-formaldehyde, are therefor undesirable since they inhibit the redispersion of fibers during repulping. The application of a lignin coating which is not cross-linked represents an improvement over such previous practices with respect to the repulpability of the treated linerboard.
The application of a lignin coating to a cellulosic product such as linerboard is described in U. S. Pat. No. 3,849,184 and Canadian Patent No. 884,264. Generally, the treatments of linerboard as described in these patents require the use of other natural or synthetic resins along with the lignosulfonate in order to achieve the strength improvements provided by the treatment or to enhance the water resistance of the treated paper product.